Typically, when one watches, for example, a foreign movie, a textual subtitle is often superimposed on the display screen along with video imagery. In video disks or ordinary TV broadcasting, video signals are transmitted in a state in which subtitles have been superimposed beforehand on the video images. In currently known systems, such as CAPTAIN and CD-G, subtitles may be transmitted as character codes or dot patterns.
CD-G is adapted to be able to record graphics utilizing subcodes and utilizing that, it is also possible to record subtitles in a CD. In the CD-G, one frame of data is composed of one byte subcode and 32 byte data, as shown in FIG. 22. In the 32 byte data, six samples of two byte data per sample are allocated to L and R channels, respectively. Accordingly, the total is 24 bytes. Then, an eight byte error correction code is added to the 24 byte audio data, resulting in the data of 32 bytes in total.
On the other hand, 98 frames of subcodes are assembled to compose one block. Among the 98 frames of subcodes, the first two frames of subcodes are allocated for sync patterns S0 and S1. Various subcode data may be recorded in the remaining 96 frames of subcodes. However, among one byte subcodes (each bit is represented by P through W), data for searching a track has been already allocated for data in P and Q channel. Then, graphic data may be allocated with 6 bits in the remaining R channel through W channel. That is, a substantial range in which the graphic data can be allocated is 6.times.96 bits.
Because one block data is transmitted with a frequency of 75 Hz, an amount of one frame of data transferred is 75.times.98 Hz. Accordingly, the bit rate for transmitting the subcodes is 7.35 kbyte/s.
FIG. 23 shows a transmission format of such graphic data. As shown in the figure, a packet is composed of 96 symbols of data, with one symbol being 6 bit data from R channel through W channel, and each packet is composed of four packs. Each pack is composed of 24 symbols from symbol 0 to symbol 23. Mode information is allocated to three bits of R, S and T in the symbol 0 and item information is allocated to three bits of U, V and W, respectively. The following modes are defined by combining such modes and items:
MODE ITEM 000 000 Zero Mode 001 000 Graphic Mode 001 001 TV-graphic Mode 111 000 User Mode
Then, an instruction is allocated to the symbol 1 and the mode and parity bits for the item and instruction are allocated to the symbols 2 and 3, so that a substantial range to which graphic data can be allocated is 12 symbols, for example, as shown in FIG. 23 among the symbols from symbol 4 through symbol 19. Parity bits for 20 symbols from the symbol 0 through symbol 19 are allocated to four symbols from symbol 20 through symbol 23.
Graphic data can be thus allocated as binary data in the range of 6.times.12 pixels of each pack in the CD-G. Because the rate of the pack is 75.times.4=300 packs/s, 300 characters may be transmitted per one second if one character is allocated in this range of 6.times.12 pixels.
Further, because one screen defined in the CD-G is 288 horizontal pixels.times.192 lines, it takes 2.56 seconds to transmit this one screen of characters as shown in the following expression: EQU (288/6).times.(192/12)/300=2.56
Still more, because different patterns have to be transmitted four times per one character pattern if a hexadecimal representation is made in each pixel, it takes 10.24 seconds which is four times of the above time.
By the way, among such conventional methods, there has been a disadvantage that an user cannot turn on or off a displayed subtitle arbitrary in the method by which the subtitle superimposed on a video image is transmitted, like the video disk and normal TV broadcasting. Further, there has been another disadvantage in them that they do not allow to prepare subtitles in a plurality of languages and to let the user select certain one.
Contrary to that, although the method in the CAPTAIN system and CD-G allows to arbitrary turn on off the display of the subtitle, it has a disadvantage that its resolution is not good enough.
That is, whereas a displayable area of one screen is 248 horizontal pixels.times.192 lines in the CAPTAIN system, component digital TV signal has a resolution of 720 horizontal pixels.times.480 lines, and it can be seen that the resolution of the former is not as good as compared to the resolution of the latter.
Furthermore, because only one bit data can be accommodated per one pixel in the CD-G, data is represented by binarizing it. Accordingly, there has been a disadvantage that such phenomena as an aliasing phenomenon by which slanted portions of a character appears in zig-zag and a flicker phenomenon by which characters flicker become significant, giving an unpleasant feeling to the user.
Although it is conceivable to convert the binary information to multi-valued information by means of a filter for example, it requires a high precision filter and hence increases the cost. The use of such filter may also degrade the background image.
If one pixel is represented in hexadecimal in the CD-G, it takes about four times the amount of time when it is displayed in binary and it becomes difficult to switch the display of subtitles quickly. Further, because the data rate is low in the method in the CAPTAIN system and CD-G, it has been difficult to display subtitles which dynamically change timewise.
Accordingly, it is an object of the present invention to solve the aforementioned problems by providing means which allows to display a high quality subtitle in accordance to a preference of an user, to switch the display of the subtitle quickly without degrading the background video image and to display the subtitle which dynamically changes timewise with less data amount.